1. FIELD
This invention relates to electronic phase shifters and, more particularly, to phase shifters which exhibit gain.
2. PRIOR ART
Phase shifters are devices designed to change the electrical phase of a signal. Two commonly used types of prior art phase shifters are the ferrite and varactor phase shifters.
The ferrite phase shifter, illustrated in FIG. 1, comprises a ferrite element 102 and a coil 103 contained within a waveguide 101. Input power, P.sub.i enters the waveguide as indicated by the arrow 105 and emerges as output power, P.sub.o at the opposite end of the waveguide, as indicated by arrow 106.
In the operation of the ferrite phase shifter, power received by the waveguide passes the ferrite element 102, where its phase is shifted in accordance with the magnetic state of the ferrite 102. The magnetic state is controlled by a current supplied to the terminals 104 of the coil 103.
This ferrite phase shifter has several drawbacks. It usually does not provide a continuous, variable electronic control of the phase over a broad frequency range, is not fast acting and it has a relatively high loss. For example, in the frequency bands from 26.5 to 110 GHz, it exhibits an insertion loss of 2 to 3.4 dB. Since power is difficult to generate at this frequency, these losses are a significant disadvantage.
FIG. 2 illustrates a varactor phase shifter in which two varactors serve as terminations in a waveguide hybrid. In this Figure, a waveguide hybrid 201 contains a wall 204 which divides the waveguide into two sections. At one end of the waveguide, varactors 202 and 203 are placed across the waveguide sections to serve as terminations. Bias voltage for the varactors is supplied to terminals 207 and 208. An opening 209, in the dividing wall 204, which is positioned close to the varactor terminations, permits power to pass from one section to the other.
In the operation of this device, input power, P.sub.i which enters one section of the waveguide, as indicated by the directional arrow 205, is incident upon the opening 209, where it divides equally between the two sections, is reflected by each varactor, and emerges from the waveguide as output power, P.sub.o, indicated by directional arrow 206. The bias applied to the varactors through terminals 207 and 208 varies the value of the reactive termination presented by the varactors and thereby produces a phase shift in the signal that is dependent upon the varactor bias. Unfortunately, the waveguide hybrid makes this type of phase shifter relatively large and a separate signal source is necessary for its use. The ferrite and varactor phase shifters are commonly used types, but other types have been developed. For example, phase shift has been achieved in oscillators by varying the oscillator supply voltage. This approach has a number of serious disadvantages. The power output of the oscillator and the frequency stability of the oscillator are usually adversely affected by varying the supply voltage.
In general, currently available electronically controlled phase shifters exhibit one or more deficiencies including power instability, relatively high loss, lack of continuous variability over a broad frequency range, slow speed and large size.
The need for improvement in phase shifters becomes clear when considering a phased array antenna, which is a primary application for these devices. A phased array antenna system is illustrated in FIG. 6. This system comprised a plurality of antenna elements, represented by elements 605A through 605D spaced apart along a line 606 at a uniform distance "d", indicated by drawing numeral 606. To produce a wave front from these antenna elements that is parallel to the line 601, all the antenna elements must be supplied with signals which are at the same frequency and phase.
To produce a wave front at an angle .phi. with the line 601, the first element 605A is supplied with a signal that is at a reference phase of zero. The second element 605B is supplied with a signal at phase angle equal to electrical degrees to ##EQU1## as indicated by drawing numeral 608. The third element is supplied with a signal at a phase angle equal in electrical degrees to ##EQU2## and so on for each succeeding element. In order to direct the beam in any direction accurately and quickly, it is necessary to precisely and rapidly set the phase of the signals at each antenna element.
A simple way of supplying the antenna elements with signals which have the required relative phase angles is shown in FIG. 7. In this Figure, four antenna elements 705A through 705D are spaced along a line 701 at a uniform distance d, indicated by drawing numeral 706. A signal source 710 supplies a signal to a divider 709 which distributes the signal to the antenna elements by way of phase shifters 708A through 708D. Each of the phase shifters is adjusted to provide the required phase shift in the signal it supplies to its antenna element. The primary disadvantages of this system are the loss in power in the phase shifters, the large range in phase shift required for each phase shifter and the relatively large size of the feed system made necessary by the separate phase shifters required for each element.